Flame-retardant polyamides

ABSTRACT

A polyamide, a copolyamide, or a polyamide blend is reacted with a monoepoxy-functional phosphorus compound at an elevated temperature to produce flame-retardant polyamides, where the amount of phosphorus compound is 5 wt % to 35 wt %, based on the total composition, i.e., the polyamide mixture.

BACKGROUND OF THE INVENTION

The invention concerns a process for producing flame-retardantpolyamides.

Synthetic polyamides (PA) are used in a variety of applications in manyareas of the industry and for everyday consumption. This is due mainlyto the good processing properties and the possibility of tailoring thesepolymers to the application. At present, just under 90% of polyamideconsumption consists of the standard types polyamide 6(poly-ε-caprolactam) and polyamide 66 (polyhexamethyleneadipamide);polyamide 11 (polyundecaneamide), polyamide 12 (poly-ε-laurinlactam),polyamide 610 (polyhexamethylenesebacamide) and polyamide 612(polyhexamethylenedodecaneamide) and copolyamides account for theremaining 10%. More than 80% of worldwide polyamide production isprocessed to fibers and fabrics; just under 20% is used in industrialapplications, in particular in automotive engineering, the electronicsindustry, the packaging sector and construction of machinery andequipment. The good mechanical properties often required industriallyare achieved with fiber reinforcement or mineral fillers. In the fieldof electrical engineering, the use of polyamides has been successfulbecause of their high insulation resistance, good tracking resistanceand solvent resistance as well as good thermo-mechanical properties, inparticular for insulation and switch parts, solenoid valves, busbars,cable mounts, coil bodies, plug connectors, and casings.

Although polyamides are self-extinguishing according to some testmethods, they lose this property after the addition of fillers such asglass fibers or pigments. For numerous applications in electricalengineering and in automotive engineering, however, reinforced,flameproof polyamide is needed. The flameproofing should offer enoughtime to rescue people and valuables in the event of a fire.

At the present time, mainly organic halogen compounds and red phosphorusare used as flameproofing agents. The halogen compounds are mainlychlorinated or brominated hydrocarbons, which are often combined withzinc compounds or antimony trioxide, the latter of which has asynergistic effect but has been found to be carcinogenic in animalexperiments. Halogen compounds have the disadvantage that they releasehighly corrosive and highly toxic degradation products such as hydrogenchloride and hydrogen bromide in a fire and they cause heavy productionof smoke; they also reduce the toughness and tracking resistance ofpolyamides. Red phosphorus is usually used in encapsulated form. Despitethe encapsulation, however, there is the danger of phosphorus fires athigh processing temperatures. This can lead to increased wear on theprocessing machines and even explosions as a result ofdisproportionation to phosphine and phosphates. Another disadvantage isthe poor electrical corrosion property of polyamide materials finishedwith red phosphorus to be flame-retardant, besides their dark color.

To avoid the disadvantages associated with halogen compounds and redphosphorus, there have been attempts for several years to developflameproofed polyamides without such flameproofing agents. For example,the use of nitrogen compounds such as dicyanodiamide (GermanOffenlegungsschrift No. 3,909,145), melamine and melamine salts (GermanOffenlegungsschrifts Nos. 3,609,341 and 4,141,861) and melamine adducts(German Offenlegungsschrift No. 3,722,118) has been proposed. To achieveadequate flame retardancy, in particular with glass fiber-reinforcedmaterials, however, very high filler levels are required, which have anegative effect on the mechanical properties. Magnesium hydroxide, whichhas also been proposed (Kunststoffe, vol. 80 (1990) pages 1107-1112),also causes a weakening of the mechanical strength, when used in therequired high concentrations; the release of water, which begins at theprocessing temperature, also causes bubbles to form. For partiallyaromatic polyamides, the use of high concentrations of polyphosphonateshas also been proposed (German Offenlegungsschrift No. 36 13 490).However, the flame retardancy achieved in that way is inadequate at highlevels of glass fiber filling; moreover, the mechanical properties ofthe polyamides are severely impaired.

Furthermore, it has already been proposed that polyamide synthesis beperformed in the presence of compounds that are incorporated into thepolymer chain during polymerization. Thus, for example, the use ofN-phosphonates and N-phosphates of ε-caprolactam has been recommendedfor polymerization of ε-caprolactam (see Journal of Applied PolymerScience, vol. 47 (1993) pages 1185-1192). In addition, the synthesis ofphosphorus-containing copolymers, e.g., by reacting 3,3'-diaminodiphenylphosphine oxide with 1,3-phenyleneisophthalamide, and the use of thesecopolymers as flameproofing agents have been recommended (see Journal ofPolymer Science, Part A, Polymer Chemistry, vol. 30 (1992) pages2521-2529). Apart from the great expense required in these cases, theresulting flame retardancy is inadequate for industrial applications.

SUMMARY OF THE INVENTION

The object of this invention is to provide a process that can be carriedout easily and inexpensively and makes it possible to synthesizepolyamides which have adequate flame retardancy for industrialapplications, do not contain any halogen compounds or red phosphorus,but nevertheless can be processed without problems, permit lightpigmentations, meet all requirements regarding electrical, chemical,mechanical and thermal properties and in particular are also suitablefor use in electrical engineering and electronics, where largequantities of fillers are required.

DETAILED DESCRIPTION OF THE INVENTION

This is achieved according to this invention due to the fact that apolyamide, a copolyamide or a polyamide blend is reacted at an elevatedtemperature with a monoepoxy-functional phosphorus compound having thestructure: ##STR1## where Z=O or S,

R¹ and R² =alkyl, O-alkyl, or S-alkyl, each with 1-4 carbons, phenyl,O-phenyl, S-phenyl, naphthyl, O-naphthyl, benzyl, or O-benzyl,

R³ =a single bond, O, alkylene, O-alkylene (O bound to P), or S-alkylene(S bound to P), each with 1-4 carbons, phenylene, or O-phenylene (Obound to P),

R⁴, R⁵ and R⁶ =H, alkyl, or hydroxyalkyl, each with 1-4 carbons, phenylor benzyl, and R⁵ and/or R⁶ may also denote --R³ --P(O)R¹ R²,

where the phosphorus compound amounts to 5-35 wt %, based on the totalcomposition, i.e., the mixture of polyamide and phosphorus compound andthe optional additive (polyamide mixture).

It has surprisingly been found that with a procedure according to theinvention, the phosphorus compound is chemically anchored to the polymerchain. Therefore, a simple process yields phosphorus-modified polyamidesthat have adequate flame retardancy, without the use of halogen or redphosphorus, and meet all the industrial requirements. It has also beenfound that the flame retardancy achieved depends only on the amount ofphosphorus anchored to the polymer chain.

The process according to the invention, which takes place at an elevatedtemperature, i.e., generally at a temperature of >150° C. (upper limit:≦350° C.) can be carried out in known heatable mixing equipment such askneaders, mixing rolls, and extruders. It is possible to preparepremixes of the polyamide and the phosphorus compound, e.g., in a drummixer or a fluid mixer, or to add the phosphorus compound directly tothe polyamide melt, distribute it therein and react it with thepolyamide, i.e., anchor it to the polymer chain. To prevent evaporationlosses when adding the phosphorus compound, it has proven advantageousto use phosphorus compounds with a molar weight of ≧150. At the sametime, other additives such as fillers, in particular glass fibers, andcolorants, antioxidants, processing aids, etc., may be incorporated anddistributed in the polyamide; filler mixtures may also be used. However,the additives may also be incorporated in advance or in a subsequentmixing operation.

In the process according to this invention, other halogen-freeflameproofing agents may also be added to the polyamide mixture toadvantage. Melamine cyanurate, melamine phosphate, and magnesiumhydroxide are preferably used as such flameproofing agents.

The phosphorus compounds used according to this invention are known perse. Reference is made to the following documents as examples:

(1) U.S. Pat. No. 2,856,369;

(2) German Offenlegungsschrift No. 19 43 712;

(3) Chemical Abstracts, vol. 56 (1962) 3511;

(4) Synthesis, 1971, pages 27 and 28;

(5) Journal of Organic Chemistry, vol. 41 (1976) pages 1165-1168;

(6) Synthesis, 1985, pages 65 and 66;

(7) Chemische Berichte, vol.124 (1991), pages 2361-2368.

The amount of phosphorus compound needed to make the polyamidesflame-retardant will depend on the chemical structure of the polyamidesand the additives. For example, polyamide 6 and polyamide 66 requireless phosphorus than polyamide 11 and polyamide 12. On the other hand,polyamide mixtures containing large amounts of fillers such as glassfibers require higher phosphorus concentrations than mixtures withoutfillers. The required phosphorus content is therefore between 1 wt % and5 wt %, based on the polyamide mixture. Since flame retardancy of thepolyamides depends only on the phosphorus concentration in the polymermatrix, different amounts of phosphorus compound are needed. Therequired amount is 5 wt % to 35 wt %, based on the polyamide mixture, asneeded, depending on the phosphorus content of the phosphorus compound.

The process according to this invention has the following advantages inparticular:

the polymer can be modified very easily and inexpensively in a mixing orextrusion process, such as that required anyway for homogenizing thepolyamides after synthesis or compounding them for various industrialpurposes;

the processing characteristics and the overall properties of thepolyamides are not affected significantly by the phosphorusmodification;

light pigmentations of the polyamides are possible with no problem;

the polyamides do not contain any volatile components, so there is nodamage or passivation of electric contacts when used in electricalengineering and electronics;

no corrosive components are released from the polyamides;

the polyamides can be recycled.

The flame-retardant polyamides produced by the process according to thisinvention are advantageously suitable as insulation materials,construction materials and casing materials in electronics andelectrical engineering. Additional fields of use include automotiveengineering, the packaging industry, machine and apparatus construction.

The invention is explained in detail below on the basis of the followingexamples.

The following components are used to produce polyamide moldingcompounds:

Component A: polyamide 6 with a relative viscosity η_(rel) of 2.9(measured on a 0.5% solution in 96% H₂ SO₄ according to ISO 307)

Component B: polyamide 66 with a relative viscosity η_(rel) of 2.7

Component C: melamine cyanurate

Component D: glass fibers with an average diameter of 10 μm

Phosphorus compounds I through VI that were used are summarized in Table1; synthesis of these compounds is described in publications (1), (4),(5), and (6).

Preparation of mixtures 1 through 10 and the corresponding moldingcompounds: Table 2 summarizes the amounts of components A through D andphosphorus compounds I through VI used to prepare the mixture. Toprepare the mixtures, the corresponding parts by weight (p.b.w.) ofcomponent A or B are plastified at 260-290° C. in a Brabender two-rollkneading machine; then the corresponding amounts of phosphorus compoundsI through VI are added in such a way that there is no lubricatingeffect. After adding the respective phosphorus compound, the mixture iskneaded for at least 5 minutes to distribute the phosphorus compoundwell and anchor it chemically to the polymer matrix. Optionally afteradding the phosphorus compound, components C and D are added, eithersimultaneously or in succession, and then the mixture is kneaded for atleast 5 minutes. After the end of the kneading process, the mixture isremoved from the kneader and cooled. The cooled melt is then ground andinjection molded to produce molded articles. Using these moldedarticles, the flame retardancy is measured according to UL 94 V (1.6 mmthick test rods), the modulus of elasticity is measured according to DIN53,457 and the impact strength is measured according to DIN 53,453. Thevalues thus obtained are summarized in Table 2.

Preparation of mixtures 11 and 12 and the corresponding moldingcompounds:

The parts by weight of polyamide 6 (component A) listed in Table 2 arepremixed with the stated parts by weight of phosphorus compound V or VIat 100° C. to 150° C. in a drum mixer or a stirred vessel and then mixedat a stock temperature of 300° C. in a twin-screw extruder (ZSK 32,Werner & Pfleiderer), then extruded into a water bath, chopped, driedand injection molded to produce molded articles. The results aresummarized in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Phosphorus compounds having the general structure                             1 #STR2##                                                                     No.                                                                              R.sup.1                                                                            R.sup.2                                                                            R.sup.3                                                                             R.sup.4                                                                          R.sup.5                                                                          R.sup.6   Z                                          __________________________________________________________________________    I  C.sub.2 H.sub.5 O                                                                  C.sub.2 H.sub.5 O                                                                  --    H  H  H         O                                          II CH.sub.3 O                                                                         CH.sub.3 O                                                                         --    CH.sub.3                                                                         H  H         O                                          III                                                                              C.sub.6 H.sub.5                                                                    C.sub.6 H.sub.5                                                                    --    CH.sub.3                                                                         H  H         O                                          IV CH.sub.3                                                                           CH.sub.3                                                                           --OCH.sub.2 --                                                                      H  H  H         O                                          V  CH.sub.3                                                                           CH.sub.3                                                                           --OCH.sub.2 --                                                                      H  H                                                                                2 #STR3## O                                          VI C.sub.6 H.sub.5                                                                    C.sub.6 H.sub.5                                                                    --SCH.sub.2 --                                                                      H  H  H         S                                          __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Composition of the mixtures and test results on the molding compounds         Example        1  2  3  4  5   6   7  8  9  10 11 12                          __________________________________________________________________________    Component A (p.b.w.)                                                                         85 87 80 85 55  --  -- 60 55 -- 90 70                          Cncnponent B (p.b.w.)                                                                        -- -- -- -- --  55  50 -- -- 55 -- --                          Component C (p.b.w.)                                                                         -- -- -- -- --  --  10 -- 10 10 -- --                          Component D (p.b.w.)                                                                         -- -- -- -- 30  30  30 30 30 30 -- --                          Phosphorus compound I (p.b.w.)                                                               15 -- -- -- --  --  -- -- -- -- -- --                          Phosphorus compound II (p.b.w.)                                                              -- 13 -- -- --  --  -- -- -- -- -- --                          Phosphorus compound III (p.b.w.)                                                             -- -- 20 -- --  --  -- -- -- -- -- --                          Phosphorus compound IV                                                                       -- -- -- 15 15  15  10 -- -- -- -- --                          (p.b.w.)                                                                      Phosphorus compound V (p.b.w.)                                                               -- -- -- -- --  --  -- 10 5  5  10 --                          Phosphorus compound VI                                                                       -- -- -- -- --  --  -- -- -- -- -- 30                          (p.b.w.)                                                                      Phosphorus content (%)                                                                       2.58                                                                             2.43                                                                             2.4                                                                              3.1                                                                              3.1 3.1 2.07                                                                             2.42                                                                             1.21                                                                             1.21                                                                             2.42                                                                             3.0                         Test results:                                                                 UL 94 V/1.6 mm V-0                                                                              V-0                                                                              V-0                                                                              V-0                                                                              V-0 V-0 V-1                                                                              V-1                                                                              V-1                                                                              V-0                                                                              V-0                                                                              V-0                         Impact strength (kJ/m.sup.2)                                                                 41 42 40 43 45  47  40 48 42 40 40 42                          Breaking elongation (%)                                                                      10 9  10 8.5                                                                              3.5 4   3.5                                                                              3.5                                                                              3  2.3                                                                              10 10                          Tensile strength (Mpa)                                                                       150                                                                              150                                                                              140                                                                              150                                                                              120 120 1tO                                                                              120                                                                              110                                                                              120                                                                              150                                                                              150                         Modulus of elasticity (N/mm.sup.2)                                                           8600                                                                             8800                                                                             8500                                                                             8800                                                                             10,000                                                                            11,000                                                                            8,000                                                                            9700                                                                             8200                                                                             8500                                                                             8800                                                                             8200                        __________________________________________________________________________

What is claimed is:
 1. A process for producing a flame-retardantpolyamide, comprising the steps of reacting a polyamide, a copolyamideor a polyamide blend at a temperature ≧150° C. with amonoepoxy-functional phosphorus compound having the structure: ##STR4##where z=O or S,R¹ and R² =alkyl, O-alkyl, or S-alkyl, each with 1-4carbons, phenyl, O-phenyl, S-phenyl, naphthyl, O-naphthyl, benzyl, orO-benzyl, R³ =a single bond, O, alkylene, O-alkylene (O bound to P), orS-alkylene (S bound to P), each with 1-4 carbons, phenylene orO-phenylene, (O bound to P), R⁴, R⁵ and R⁶ =H, alkyl or hydroxyalkyl,each with 1-4 carbons, phenyl or benzyl, and R⁵ and/or R⁶ may alsodenote --R³ --P(O)R¹ R²,wherein the phosphorus compound amounts to 5-35wt %, based on the total composition.
 2. The process according to claim1, wherein the additive is incorporated into the polyamide.
 3. Theprocess according to claim 2, wherein an additive is a filler.
 4. Theprocess according to claim 3, wherein glass fibers are used as thefiller.
 5. The process according to claim 1, wherein a halogen-freeflameproofing agent is added to the polyamide.
 6. The process accordingto claim 2, wherein a halogen-free flameproofing agent is added to thepolyamide.
 7. The process according to claim 3, wherein a halogen-freeflameproofing agent is added to the polyamide.
 8. The process accordingto claim 4, wherein a halogen-free flameproofing agent is added to thepolyamide.
 9. The process according to claim 5, wherein melaminecyanurate, melamine phosphate, or magnesium hydroxide is used as theflameproofing agent.
 10. The process according to claim 6, whereinmelamine cyanurate, melamine phosphate, or magnesium hydroxide is usedas the flameproofing agent.
 11. The process according to claim 7,wherein melamine cyanurate, melamine phosphate, or magnesium hydroxideis used as the flameproofing agent.
 12. The process according to claim8, wherein melamine cyanurate, melamine phosphate, or magnesiumhydroxide is used as the flameproofing agent.
 13. A insulation materialcomprising the flame-retardant polyamide, according to claim
 1. 14. Aconstruction material comprising the flame-retardant polyamide,according to claim
 1. 15. A casing material comprising theflame-retardant polyamide, according to claim 1.